With the advent of semiconductor devices having increasingly large component densities, the removal of heat generated by the devices has become an increasingly challenging technical issue. In the last decade, the frequency of operation of CMOS devices has increased by an order of magnitude. The resulting microprocessor power dissipation has likewise gone up by an order of magnitude. While the input voltage and capacitance of devices has decreased, the number of devices on a typical microprocessor die continues to increase rapidly as processing efficiency is sought. Moreover, device miniaturization has led device designers to integrate previously separate components, such as those used to create a cache, into the microprocessor die.
This consolidation of devices has resulted in high CPU core power densityxe2x80x94e.g., 50% of a 20 mm by 20 mm microprocessor die may contain the CPU core, with the rest being cache. Extrapolating the changes in microprocessor organization and the device miniaturization, one can project future power dissipation requirements of 100 W from a 1 cm by 1 cm core CPU surface area within the easily foreseeable future. Furthermore, typical processor boards can, in some instances, include multiple CPU modules, application-specific integrated circuits (ICs), and static random access memory (SRAM), as well as a dc-dc converter, all of which have increasing power dissipation requirements, thereby increasing the total power dissipation level needed by computer systems.
Adding to the complexity, modern systems using such high-dissipation chips frequently have a variety of chips requiring different levels of cooling, only some of which are extreme. Depending on an electronic system""s design, components containing these chips can be located throughout a system, and might not be easily dealt with using either simple cooling means, or using cost-efficient high-dissipation cooling means directed only to the chips requiring their capabilities.
Heat sinks can be used to increase the heat-dissipating surface area of heat-producing devices. However, heat sinks, are typically characterized by a mechanical interface to their cooled devices, which commonly leads to interference in the heat flow, and can lead to very high thermal resistance. Indeed, the bulk of the available thermal budget for cooling, typically a 45xc2x0 C. temperature differential between the chip temperature and the ambient temperature, will commonly be used up by this interface. The mechanical interface can also lead to uneven cooling. This is further complicated by the non-uniform power distribution on many chips, which often results when different components are integrated onto a single chip.
To deal with these difficulties, innovative ways have been developed to reduce chip-to-heat sink thermal resistance. Included among the cooling methods for semiconductors are free-flowing and forced-air convection, free-flowing and forced-liquid convection, pool boiling (i.e., boiling a liquid cooling fluid off of a submerged device), and spray cooling (i.e., boiling a liquid cooling fluid off of a device being sprayed with the liquid). Because liquids typically have a high latent heat of vaporization, these latter two methods provide for a high heat-transfer efficiency, absorbing a large quantity of heat at a constant temperature.
The use of these boiling/vaporizing methods is limited to a maximum power density, the critical heat flux (CHF). At higher densities, the vaporized cooling fluid forms a vapor barrier insulating the device from the liquid cooling fluid, thus allowing the wall temperature of the device to increase greatly. This phenomenon is referred to as dry-out. When a coolant is properly sprayed, it can disperse such a vapor layer, and its CHF can be well over an order of magnitude higher than the CHF of a pool-boiling system. This high CHF is preferably a uniform spray, and should match the power dissipation requirements of the device. Thus, spray cooling presently provides the most efficient cooling for a heat-generating device, such as a semiconductor device.
Typically, the cooling fluid used for spray cooling has a relatively low boiling point (in relation to the operating temperature of the device), which is the temperature that the sprayed device is cooled toward. Most preferably, the cooling fluid is inert to the heat source. For semiconductor devices, low boiling point fluids such as 3M(copyright) FC-72 (FED. CIR.-72, i.e., FLUORINERT(copyright), sold by 3M(copyright) Corporation), 3M""s Novec line of fluids (HFE 7100, etc., sold by 3M(copyright) Corporation) or PF-5060 are among a number of known suitable cooling liquids.
The nozzle design is a key component of spray cooling. Pressure assisted and gas assisted nozzles are known designs. However, these types of nozzles are limited in their ability to control the rate at which they spray. Therefore, they can cause xe2x80x9cpoolingxe2x80x9d (i.e., a buildup of liquid on the cooled device due to excessive spray rates). Also, a fine, localized control of the spray rate, direction and/or location is not generally available.
For pressure-assisted spraying, consistent, controlled spraying requires one or more high pressure pumps that provide a precise pressure to pump the liquid through a nozzle, even at varying flow rates. Both the distribution and the flow rate of the sprayed liquid can change with variations in the driving pressure and/or small variations in the nozzle construction. Thus, the cooling system is a sensitive and potentially expensive device that can be a challenge to control.
For gas atomizing, consistent, controlled spraying requires a pressurized gas that is delivered to a spray head design in a precise manner. Because the gas must be pressurized separately from the cooling fluid, such systems are not typically closed systems. The gas must be bled out for the condenser to run efficiently. Furthermore, both the distribution and the flow rate of the cooling fluid can change with variations in the gas pressure. Thus, the cooling system is a sensitive and potentially expensive device that can be a challenge to control.
Typically, current sprayer designs employ either pressurized liquid spraying or pressurized gas atomizing. A number of factors affect the performance of spray cooling, thus affecting the heat transfer coefficient h and/or the CHF. It is commonly understood that surface roughness and wettability of the sprayed component are two of these factors, and the orientation of the surface being sprayed can be a third. In particular, it is believed that h is higher for rough surfaces when using a pressurized liquid spray, and for smooth surfaces when using gas atomizing. Surfaces with decreased wettability appear to have a marginal increase in h.
Critical to consistent, controlled cooling is the controlled application of the liquid cooling fluid in a desired distribution, flow rate, and velocity. For example, at a low mass flow rate, CHF and h increase with the mass flow rate. However, at a critical mass flow rate, the advantages of increased mass flow are diminished due to pooling and/or due to a transition to single phase heat transfer. Thus, a spray cooling system is preferably operated uniformly at a mass flow rate defined at a point before the critical mass flow rate is reached. All of these factors make critical the design of the sprayer, i.e., the design of the nozzle and its related spray devices.
Also important to the cooling system design is its operating temperature. In particular, it is desirable to configure the system to operate at a high h, which will occur with a design temperature above the boiling temperature and below a temperature that will dry out the sprayed coolant. The amount of heat to be dissipated must be less than the CHF.
Typically, in implementing a spray cooling system, a spray cooling sprayer is mounted within a container that also contains a printed circuit board carrying one or more chips. The printed circuit board has leads that electrically connect to systems outside of the container via air- and water-tight connections. Such systems can be large, and can be expensive to build and maintain.
Accordingly, there has existed a need for a small, accurate, reliable and cost-efficient spray cooling system that can be used to support complex processing systems having one or more high-dissipation devices. The present invention satisfies these and other needs, and provides further related advantages.
In various embodiments, the present invention solves some or all of the needs mentioned above by providing a package-level cooling system that efficiently operates on a package of one or possibly more high-dissipation devices with spacial and cost efficiency.
The system for cooling a semiconductor device with a cooling fluid, of the invention, features a body configured to be retained against a portion of a package, which may be a component made of one or more exposed semiconductor devices (i.e., chips) on a component substrate, a component made of one or more chips on a component substrate covered by an encapsulant or lid, or even one or more exposed or covered components mounted on a component substrate such as a printed circuit board. The body is configured to form a spray chamber in which the cooling fluid can be sprayed into thermal contact with a portion of the chip(s), either directly or indirectly. Advantageously, designs embodying this feature typically provide for simple assembly of the cooling system and the semiconductor device, and take up little space.
The invention also features the use of incremental sprayers, being configured to eject incremental amounts of cooling fluid onto the chips in response to a control signal. These incremental sprayers are preferably of a thermal inkjet-type design, typically providing evaporative cooling by a continuous xe2x80x9cmistxe2x80x9d on a high-power dissipation-surface. This feature can provide for excellent flow rate control in most designs, as well as operating reliably with few or no mechanical movements in the spray system.
An integral condenser and reservoir, configured to receive and cool (i.e., remove heat from) cooling fluid from an outlet of the spray chamber, and configured to feed cooling fluid to an inlet of the sprayer, is also featured in the invention. Advantageously, this feature can provide for a more self-contained system that does not rely on thermodynamic mechanisms of the processing system. It can also provide for cost and spacial efficiency in the manufacture and use of the cooling system.
Other features and advantages of the invention will become apparent from the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The detailed description of particular preferred embodiments, as set out below to enable one to build and use an embodiment of the invention, are not intended to limit the enumerated claims, but rather, they are intended to serve as particular examples of the claimed invention.